There are two kinds of technology that are widely used in implementing door/window intrusion sensors. One includes a combination of a reed switch with a magnet (mechanical contacts). The other includes a source, an infrared (IR) sensor and a reflector. Unfortunately, each of these methods could be defeated by a person having limited knowledge of the sensors.
Mechanical contacts can be easily defeated from the outside by using an additional magnet to keep the reed switch actuated while the window or door is being opened.
Known intrusion sensors based on IR transmission have constantly transmitted the IR. Such devices may be defeated by shining a light, such as flashlight, at the IR sensor, or using a thin mirror as a reflector to defeat the device.